Unit 3 Topic 2

Pioneer species

A pioneer species is one of the first species to colonize a previously barren, disturbed, or newly formed environment (such as bare rock, volcanic ash, sand dunes, or a burnt/cleared area) during the early stages of ecological succession, before soil or a stable community has developed.

Photosynthesis and Energy Conversion

Light energy is converted into chemical energy in an ecosystem through photosynthesis: producers (plants, algae, cyanobacteria) use chlorophyll to capture light energy and convert it into chemical energy stored in glucose, using carbon dioxide and water as raw materials (6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂).

Biomass Production

Biomass is the total mass of living (or once-living) organic material in an area, usually measured as dry mass per unit area. It's produced when organisms build organic molecules through photosynthesis or by consuming and assimilating other organisms' tissue.

Biomass Interaction with Carbon Cycle

Biomass production interacts with the carbon cycle through photosynthesis, which removes CO₂ from the atmosphere and fixes it into organic carbon compounds; respiration releases some carbon back as CO₂, and decomposition further recycles carbon back into the atmosphere.

Energy Loss in Ecosystems

At every trophic level, energy is used for cellular respiration for life processes and is released as heat, resulting in roughly 10% energy transfer to the next trophic level.

One-way Energy Flow

Energy flow through an ecosystem is described as 'one-way' because energy cannot be recycled after conversion to heat, requiring a constant input of solar energy, whereas matter is cycled repeatedly.

Energy Flow Diagram

An energy flow diagram shows the quantity of energy entering and moving through each trophic level, detailing inputs (from the level below) and outputs (energy passed to the next trophic level, lost as heat, and lost to decomposers).

Ecological Efficiency Calculation

Ecological efficiency (%) is calculated as (Energy at trophic level n+1 / Energy at trophic level n) × 100, typically around 10% between trophic levels.

Gross Primary Productivity (GPP)

The total rate at which producers convert light energy into chemical energy per unit area per unit time before any losses.

Net Primary Productivity (NPP)

The energy/biomass remaining after subtracting producers' respiration from GPP: NPP = GPP − R, representing the energy available to consumers.

Consumer Productivity

Gross (secondary) productivity is the total energy assimilated by consumers; net (secondary) productivity is what remains after subtracting energy used for respiration.

Ecological Pyramids Comparison

An ecological pyramid of energy is always upright because energy is lost as heat at each trophic level, while pyramids of numbers or biomass can be inverted.

Solar Energy Loss Forms

Reflection, absorption, and radiation are forms of solar energy loss; reflection bounces light off surfaces, absorption converts energy to heat, and radiation re-emits energy back into the environment, limiting chemical energy capture.

Water Cycle Processes

Key processes of the water cycle include evaporation, transpiration, condensation, precipitation, and runoff/infiltration.

Carbon Cycle Processes

Key processes of the carbon cycle involve photosynthesis, respiration, decomposition, combustion, and long-term storage.

Nitrogen Cycle Processes

A: (1) Nitrogen fixation — N₂ (gas) converted to NH₃/NH₄⁺ (ammonia/ammonium) by bacteria or lightning → (2) Nitrification — NH₄⁺ converted to NO₂⁻ (nitrite), then NO₂⁻ converted to NO₃⁻ (nitrate) by nitrifying bacteria → (3) Assimilation — plants absorb NO₃⁻ (and sometimes NH₄⁺) and build proteins/nucleic acids, passed along the food chain through feeding → (4) Ammonification — decomposers break down dead matter/waste, releasing nitrogen back into the soil as NH₃/NH₄⁺ → (5) Denitrification — denitrifying bacteria convert NO₃⁻ back into N₂ (gas), returning it to the atmosphere.

Necessity of Nitrogen Fixation

Nitrogen fixation is necessary because most organisms cannot use N₂ gas directly; nitrogen-fixing bacteria convert N₂ into ammonia, making it usable for plants.

Decomposers' Role

Decomposers link matter cycles to energy flow by breaking down organic matter and recycling nutrients while releasing energy as heat.

Predation Definition

Predation is an interaction where one organism (the predator) hunts and consumes another (the prey), benefiting the predator and harming the prey.

Competition Types

Competition occurs when organisms compete for limited resources. Intraspecific competition is among individuals of the same species, while interspecific competition is between different species.

Mutualism Example

Mutualism is a relationship where both species benefit, such as bees pollinating flowers while obtaining nectar.

Commensalism Example

Commensalism is a relationship where one species benefits and the other is neither helped nor harmed, such as barnacles on a whale.

Parasitism Example

Parasitism is a relationship where the parasite benefits at the host's expense, like tapeworms in mammal guts.

Ecological Niche

An ecological niche is the full role and position a species occupies within an ecosystem, encompassing its habitat and resource use.

Fundamental vs. Realised Niche

The fundamental niche is the potential range of conditions a species could use, while the realized niche is the actual range used given competition and other interactions.

Competitive Exclusion Principle

The competitive exclusion principle states that two species with identical niches cannot coexist indefinitely due to competition for limited resources.

Resource Partitioning

Resource partitioning is when similar species evolve to use resources slightly differently to minimize competition and coexist.

Keystone Species Role

A keystone species has a large effect on community structure and diversity relative to its abundance, and its removal can lead to ecosystem collapse.

Example of Keystone Species

Sea otters are a classic keystone species, controlling sea urchin populations to maintain kelp forest ecosystems.

Identifying Keystone Species

Food web data can identify keystone species by highlighting those with many connections or whose removal causes large changes in the ecosystem.

Inferring Species Interactions

Food web/population data can infer interactions by examining trends, resource use, and co-occurrences among species.

Predicting Outcomes of Species Removal

To predict the outcomes of removing a species from a food web, analyze its direct connections and potential flow-on effects.

Effects of Overexploitation

Overexploitation can reduce species populations below sustainable levels, disrupt food webs, and decrease biodiversity.

Effects of Habitat Destruction

Habitat destruction reduces space and resources, leading to population declines and loss of biodiversity.

Effects of Monocultures

Monocultures decrease species richness and genetic diversity, making ecosystems more vulnerable to pests and diseases.

Effects of Pollution

Pollution can directly harm species, cause bioaccumulation, alter abiotic factors, and shift community composition.

Biotic Factors and Carrying Capacity

Changes in biotic factors, such as new predators or food source loss, can alter an ecosystem's carrying capacity.

Abiotic Factors and Carrying Capacity

Changes in abiotic factors, like resource availability or environmental conditions, can raise or lower an ecosystem's carrying capacity.

Climatic Events and Carrying Capacity

Climatic events can drastically reduce available resources and carrying capacity or temporarily increase it after favorable conditions.

Ecological Succession Defined

Ecological succession is the gradual change in species composition over time as early species modify the environment for new species.

Primary vs. Secondary Succession

Primary succession starts on lifeless substrates and is slow; secondary succession occurs in disturbed areas where soil remains and is faster.

Effective Pioneer Species Traits

Pioneer species are effective colonizers due to wide dispersal, tolerance of harsh conditions, rapid growth, and soilformation capabilities.

Species Interactions in Succession

Species interactions through succession change from minimal competition to increased complexity as community structure develops.

Abiotic Changes in Succession

Through succession, abiotic conditions become more favorable as pioneer species improve soil, nutrient, and water retention.

Shift from r- to K-selected Species

Early succession is dominated by r-selected species that are fast-growing, while K-selected species dominate later stages.

Biodiversity Changes Through Succession

Biodiversity generally increases during succession as new niches develop, although it may plateau near the climax stage.

Biomass Changes Through Succession

Biomass increases through succession as larger, longer-lived organisms replace pioneer species, culminating in the climax community.

What is the difference between intraspecific and interspecific competition?

Intraspecific competition occurs between individuals of the same species (e.g. two lions competing for the same prey). Interspecific competition occurs between individuals of different species (e.g. a lion and a hyena competing for the same prey). Both involve competing for the same limited resource (food, space, light, mates), but intraspecific competition is often more intense, since individuals of the same species have identical resource needs and niches.

Symbioses

close, long-term interaction between two different species living together, which can be mutualistic (both benefit), commensal (one benefits, the other unaffected), or parasitic (one benefits at the other's expense).

Describe how quadrats could be used with random sampling to investigate the plant species diversity of a 100 m × 100 m paddock.

4 m

“Mark out the paddock as a grid and use a random-number generator to choose coordinates for each quadrat. Place 20 quadrats of 2 m × 2 m at these positions, as a larger number is more representative. In each quadrat, identify each plant species and count the individuals or estimate the percentage cover. Use the combined data to calculate species richness and Simpson’s index of diversity.”

Random sampling

A sampling method where sample positions are chosen purely by chance so every location has an equal probability of being selected. It is preferred when the area is fairly uniform without obvious zones or environmental gradients, eliminating researcher selection bias and avoiding alignment with repeating habitat patterns. Bias is minimised by using a random-number generator for grid coordinates and collecting a large number of samples to prevent chance distortion.

Systematic sampling

A sampling technique where samples are collected at regular, fixed intervals along a line or grid. It is preferred when investigating changes along an environmental gradient (such as from water to dry land) because it provides even coverage without clustering. Bias is minimised by selecting the initial starting point randomly, taking a large number of samples, and choosing intervals that avoid coinciding with repeating environmental features.

Stratified sampling

A sampling strategy where an area is partitioned into distinct zones or habitats (strata) and each is sampled separately in proportion to its total area. It is preferred when an area features discrete habitat types with distinct communities (such as beach and mangrove) to ensure smaller zones are not missed. Bias is minimised by allocating sample numbers proportionally and placing sample points randomly within each stratum using a random-number generator.

Quadrats

Square frames placed on the ground within which individuals are counted or percentage cover is estimated. They are preferred for plants and sessile or slow-moving organisms to determine population density and cover in a defined area. Bias is minimised by positioning frames using random coordinates, establishing clear inclusion criteria for boundary organisms (such as only counting those rooted inside), and sampling with sufficient replication.

Line transect

A tape or line positioned across an environmental gradient where organisms touching the line are recorded at regular intervals or continuous points. It is preferred as a rapid, low-effort technique for recording changes in species presence across large distances or gradients. Bias is minimised by selecting the line position randomly, standardising contact criteria, and replicating across multiple lines.

Belt transect

A strip of defined width marked along a gradient that is sampled continuously or at set intervals using quadrats. It is preferred over a single line when target species are sparse or clustered and quantitative abundance data across a gradient is required. Bias is minimised by maintaining consistent strip width and quadrat sizes, replicating transects, and adhering to strict counting rules for organisms on the edge.

Capture-recapture

A sampling method for mobile animals involving capturing individuals, applying a harmless mark, releasing them to disperse, and collecting a second sample to estimate population size via the Lincoln index: $N = \frac{M \times n}{m}$. Bias is minimised by applying non-toxic marks that do not impair survival or increase predation risk, allowing sufficient time for random mixing, and using identical capture techniques across both sampling events.